Various implantable medical devices have been developed for treating ailments in the vascular system. Vaso-occlusive devices have been used extensively in closing regions of the vascular system. These devices are especially useful in treating aneurysms. Vascular aneurysms are formed as a result of a weakening in the wall of an artery and subsequent ballooning of the artery wall. Aneurysms are often a site of internal bleeding and, catastrophically, the site of strokes. A variety of different embolic agents are known to be suitable for treatment of such aneurysms. These treatments are commonly known as "artificial vaso-occlusion."
One such class of embolic agents includes injectable fluids or suspensions, such as microfibrillar collagen, various polymeric beads, and polyvinyl alcohol foam. These polymeric agents may additionally be crosslinked, sometimes in vivo to extend the persistence of the agent at the vascular site. These agents may be introduced into the vasculature through a catheter. After such introduction, materials there form a solid space-filling mass. Although some such agents provide for excellent short-term occlusion, many are thought to allow vessel recanalization due to absorption of polymer into the blood. Another procedure in which a partially hydrolyzed polyvinyl acetate (PVA) is dissolved in ethanol solvent and injected into a desired vascular site is found in Park et al. (U.S. patent application Ser. No. 08/734,442, filed Oct. 17, 1996, for "LIQUID EMBOLIC AGENTS").
Other materials, such as hog hair and suspensions of metal particles have also been suggested and used by those wishing to form occlusions.
Other materials, including polymeric resins, typically cyanoacrylates, are also employed as injectable vaso-occlusive materials. These resins are typically mixed with a radio-opaque contrast material or made radio-opaque by the addition of a tantalum powder. These materials are difficult to use in that the placement of the mixture in the body may be a problem. These materials cross-link within the human body. Inadvertent embolisms in normal vasculature, due to the inability of controlling the destination of the precursor resinous materials, is not uncommon. The material is also difficult or even impossible to retrieve once it has been placed into the vasculature.
Over the past few years, advancements in the artificial occlusion of vessels and aneurysms has occurred mostly due to the improvements in delivery and implantation of metal coils as vaso-occlusive devices. Implantable metal coils that are useful in artificial occlusion devices in vasculature lumens or aneurysms are herein referred to as "vaso-occlusive coils."
Vaso-occlusive coils are generally constructed of wire, usually made of a metal or metal alloy, that is first wound into a helix. Many such devices are introduced to the selected target site through a catheter in a stretched linear form. The vaso-occlusive device may assume an irregular shape upon discharge of the device from the distal end of the catheter. A variety of vaso-occlusive coils and braids are known. For instance, U.S. Pat. No. 4,994,069, to Ritchart et al., shows a flexible, preferably coiled, wire for use in small vessel vaso-occlusion. These coils are described as being between 0.010 and 0.030 inches in diameter. The wire used to make up the coils may be, for instance, 0.002 to 0.006 inches in diameter. Tungsten, platinum, and gold threads or wires are said to be preferred. These devices may be used to fill aneurysms.
It is common that these vaso-occlusive devices be delivered through microcatheters such as the type shown in U.S. Pat. No. 4,739,768, to Engelson. These microcatheters track a guidewire to a point just proximal or within the desired occlusion site. The vaso-occlusive coils are then advanced through the microcatheter, once the guidewire is removed, and out the distal end hole so to at least partially fill the selected site and create occlusion within the aneurysm.
In addition to the various types of space-filling mechanisms and geometries of vaso-occlusive coils, other particularize features of coil designs, such as mechanisms for delivering vaso-occlusive coils through delivery catheters and implanting them in desired occlusion sites, have also been described. Examples of such vaso-occlusive devices based upon their delivery mechanisms include pushable coils (see Ritchart et al., discussed above), mechanically detachable vaso-occlusive devices (see U.S. Pat. No. 5,261,916 to Engelson or U.S. Pat. No. 5,250,071 to Palermo), or electrolytically detachable vaso-occlusive devices (see U.S. Pat. Nos. 5,122,136 and 5,354,295 to Guglielmi et al.).
Each of the devices described above may be used in the treatment, by occlusion, of aneurysms. As noted above, aneurysms present particularly acute medical risks due to the dangers of potential rupture of the thin wall inherent in such a vascular anomaly. Occlusion of aneurysms by the use of vaso-occlusive coils without occluding the adjacent artery is a special challenge.
As noted above, the use of vaso-occlusive coils in treating aneurysms is widespread. These vaso-occlusive devices are placed in an aneurysm by the use of a microcatheter. The distal end of the microcatheter is advanced into the mouth of the aneurysm. The vaso-occlusive coil is then advanced through and out of that microcatheter. After, or perhaps during, delivery of such a coil into the aneurysm, there is a specific risk that a portion of the coil might migrate out of the aneurysm entrance zone and into the feeding vessel. The presence of such a coil in that feeding vessel may cause the undesirable response of causing an occlusion there. Also, there is a quantifiable risk that the blood flow in the vessel and the aneurysm may induce movement of the coil farther out of the aneurysm, resulting in a more thoroughly developed embolus in the patent vessel.
Furthermore, one type of aneurysm, commonly known as a "wide-neck aneurysm" is known to present particular difficulty in the placement and retention of vaso-occlusive coils. Wide-neck aneurysms are herein referred to as aneurysms of vessel walls having a neck or a "entrance zone" from the adjacent vessel, which entrance zone has a diameter of either (1) at least 80% of the largest diameter of the aneurysm; or (2) is clinically observed to be too wide effectively to retain vaso-occlusive coils that are deployed using the techniques discussed herein.
Furthermore, vaso-occlusive coils lacking substantial secondary shape strength may be difficult to maintain the position within an aneurysm no matter how skillfully they are placed.
There are a few disclosed devices for maintaining the presence of vaso-occlusive coils within an aneurysm. One such device is shown in U.S. Ser. No. 08/690,183, filed Jul. 26, 1996 for "ANEURYSM CLOSURE DEVICE ASSEMBLY" (Attorney Docket No. 29025-20162.00). That patent describes a number of devices which are to be placed within the lumen of a feed vessel exterior to the aneurysm. It may be used to retain coils within the aneurysm cavity. One highly desired way of using the device is this: the retainer device is released into the vessel exterior to the aneurysm. The device is held in place via the presence of radial pressure on the vessel wall. After the device is released and set in an appropriate place, a microcatheter is inserted into the lumen so that the distal end of the catheter is inserted into the aneurysm cavity. One or more vaso-occlusive devices is then introduced into the aneurysm cavity. The retainer device maintains the presence of the vaso-occlusive devices within the aneurysm whether it is a large-mouth aneurysm or not.
Other stents or grafts which may be used to seal the mouth of an aneurysm are shown, e.g., in U.S. Pat. No. 4,820,298, to Leveen et al.; U.S. Pat. No. 5,258,042 to Metha; and others. None of these devices are of the same configuration as that described herein.